Typically, a known air conditioning device installed in a building including multiple rooms has a refrigerant circuit to which an outdoor unit and multiple indoor units are connected for providing a vapor compression refrigeration cycle. (See, for example, PATENT DOCUMENT1.)
When a compressor of the refrigerant circuit is activated, portion of refrigerating machine oil, stored in the compressor for lubricating a compression mechanism and a bearing in the compressor, flows out of the compressor together with a refrigerant and circulates in the refrigerant circuit. Here, in liquefied portion of the refrigerant in the refrigerant circuit, the refrigerating machine oil flows in the circuit together with the refrigerant; however, in gaseous portion of the refrigerant, portion of the refrigerating machine oil adheres to an interior surface of a heat exchanger tube of a heat exchanger and an interior surface of a refrigerant pipe. Hence, portion of the refrigerating machine oil flowing into the refrigerant circuit fails to return to the compressor, and continuous operation of the compressor reduces an amount of refrigerating machine oil stored in the compressor. Then, when the amount of the stored refrigerating machine oil becomes smaller than a predetermined amount, the compressor tends to develop a lubrication-related malfunction.
Thus, this kind of air conditioning device typically performs oil collecting operation which involves forcibly returning, to the compressor, refrigerating machine oil that stays in the refrigerant circuit and fails to return to the compressor. In the oil collecting operation, a flow rate of the gaseous refrigerant is usually increased so that the refrigerating machine oil is caught by the flow of the refrigerant and the caught refrigerating machine oil is sucked into the compressor together with the refrigerant.
The oil collecting operation is performed after each elapse of a time period set by a timer. Moreover, of an interconnecting pipe connecting the outdoor unit and an indoor unit, a main pipe is to be connected to the outdoor unit, and a branch pipe is to branch off from the main pipe and be connected to each of the indoor units. The oil collecting operation is also performed in the following case: When the flow rate of the refrigerant in the main pipe is short, the refrigerating machine oil is determined not to return to the compressor and the amount of refrigerating machine oil not returning to the compressor (the amount of lost oil) is calculated. When a value obtained by integrating the calculated values becomes greater than a certain amount, the oil collecting operation is performed.